Making use of a charged particle beam apparatus, charged particle beam is scanned on a sample two-dimensionally in the horizontal and vertical directions, a secondary signal ejected from a surface sample region on which charged particle beam comes incident is detected and a surface structure in the surface region is observed as a two-dimensional image by replacing an amount of the secondary signal with a concentration value of the image data for each of the coordinates in the two-dimensional image which are associated with the scan coordinates for the charged particle beam. In this charged particle beam apparatus, the scan speed of the charged particle beam is determined according to such factors as a material property of the sample, the objective of the observation and the specification of a secondary signal detector.
When the scan speed is high, an integration time of the detection signal is so short that a S/N ratio becomes low while a response to a time-to-time change of a sample is quick and the charged particle beam has a smaller effect on destruction, contamination and charging of the sample because the time for the charged particle beam being radiated on a point on the sample is short.
On the other hand, when the scan speed is low, the integration time of the detection signal is so long that images with the high S/N ratio are obtained while the charged particle beam has a larger effect on destruction, contamination and charging of the sample.
Therefore scanning is usually performed at a high speed while a desired observation area is being searched and at a low speed when the desired observation area is scanned to take and record a high resolution image.
Moreover when such a sample as is easily destructed, contaminated or charged by the charged particle beam being radiated is observed, it is possible to take the high scanning speed to reduce the above mentioned effect. However, there are other countermeasures than taking the high scanning speed, such as setting a lower voltage to an acceleration voltage and reducing an electrical current flowing through the charged particle beam that is radiated on the sample by making a probe diameter of the charged particle beam smaller.
Other factors to determine the scanning speed of the detector than the above mentioned are properties of the detector. That is, the scanning speed of the detector is dependent on a response speed for the secondary signal to be detected by the detector and converted by the detector to an electrical signal and a response speed for the detection signal is processed through an amplification circuit. The larger the gain for the amplification circuit, the slower the response time for the amplification circuit.
If the detector response speed and the amplification circuit response speed are low, the frequency band for the detection signal is restricted while it is being detected and amplified, however fast the charged particle beam is scanned.
When a detection signal which is obtained while a microstructure of the sample is being scanned and has high frequency components is inputted to the detector circuit above mentioned, an impulse response is created and an output signal for a time t for which the impulse response converges becomes a summation of the detection signal and the impulse response.
The image of the output signal above mentioned flows in the direction in which the charged particle beam is scanned and is blurred so much that it is impossible to identify a sample structure. Accordingly it is impossible to observe a sample at a high scan speed with a high gain if a detector whose response speed is relatively low is used.
The detector is selected according to the secondary signal to be detected and such conditions as atmosphere in the sample chamber and detection sensitivity. Therefore, as is the case with the above mentioned, a detector whose response speed is low is used to observe a sample at a relatively low scan speed, depending on the observation condition. However there is a problem that it is hardly possible to make an observation at a high scan speed in order to look for an area in which a point of interest exists or to reduce the damage of the sample in this case.
There is a conventional technology described in WO2003/044821 with which the image deterioration, which occurs due to the charged particle beam being scanned and the secondary signal ejected during the scanning is compensated for making use of the images obtained after plural times of scanning. WO2003/044821 discloses a technology to correct a drift of the observed image with time elapsing, which occurs due to such factors as the change in stability of the charged particle beam, deformation of the sample and a shift of the sample stage. That is, WO2003/044821 discloses a technology to correct the image position by calculating an image shift based on images obtained after plural times of scanning and a technology to improving the S/N ratio of the image by integrating images which are obtained by plural times of high speed scanning and whose image shifts are corrected according to the above mentioned technology. However, looking to deterioration of the images that are obtained by high speed scanning, the waveform of the output signal is not deformed by an amount of output signals or noises included in the output signal, but deformed by the property of the detector. Therefore this deterioration is not prevented by such measures as improving the S/N ratio by integration.